Methane air-sea fluxes in two fjords in West Spitsbergen

Methane is an important greenhouse gas and plays a key role in the climate feedback mechanism. Numerous publications projected near-term catastrophic climate scenarios attributed to massive methane release from thawing of subsea permafrost and dissociation of gas hydrates. Higher seawater temperatures driven by the current anthropogenic warming event could potentially increase the susceptibility of submarine permafrost and destabilize methane hydrates. In situ measurements from high arctic shelves, where marine permafrost has been exposed to warmer bottom temperatures for the last couple of decades, are critical to better quantify sea methane fluxes. Samples were collected throughout the water column of two Arctic fjords in West Spitsbergen (Nordfjorden and Tempelfjorden), where maximum depths reach 95 m and 195 m subsequently, in the spring of 2017. In Nordfjorden (medium depth fjord), high methane concentrations were observed (>40 nmol L^-1) below the pycnocline near the seabed in the vicinity of areas where pockmarks and gas flares are well documented. However, only <10 nmol m^-2·s^-1 reached the atmosphere due to a combination of physical and biological processes. The majority of the methane that migrates from the seafloor into the water column is diffused into the ocean and then oxidized by microbial activity. In Tempelfjorden (shallow fjord), methane concentrations were evenly distributed throughout the water column (∼12.5 nmol L^-1), nevertheless, higher methane ocean-atmospheric fluxes were recorded (∼16 nmol m^-2·s^-1) resulting from higher wind velocities. The findings of this study stress the complexity of the physical and biological processes that govern methane interactions between ocean and atmosphere. Aside from the subsea methane sources, localized physical and biological processes play a substantial role in the fate of sea-air methane fluxes in high Arctic shelves.